JPH01127085A - Method for anticorrosion coating of steel pipe - Google Patents

Abstract

PURPOSE:To improve the close bonding strength, water resistance and hot water resistance of a film, by successively forming an epoxy paint layer and an adhesive resin layer consisting mainly of modified polyolefin having a specific composition to the outer surface of a steel pipe. CONSTITUTION:A steel pipe to which surface treatment is preliminarily applied is preheated to 150 deg.C or more and painted with epoxy powder paint to form the first layer. While the flowability of the first layer is not lost, the first layer is coated with an adhesive resin composition consisting mainly of modified polyolefin, which is a molten mixture of modified polyolefin containing a functional group being one or more kind of a carboxylic group or a dicarboxylic anhydride group and an epoxy resin having one internal epoxy group in one molecule thereof and one or more terminal epoxy group, to form the second welded layer. Further, a polyolefin layer is welded and applied to the second layer to form the third layer. By this method, an anticorrosion coating layer is formed to the steel pipe.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for anticorrosive coating of steel pipes, and more specifically, the present invention relates to a method for anticorrosive coating of steel pipes, and more specifically, a first layer of epoxy powder coating on the outer surface of the steel pipe, and an adhesive resin mainly composed of modified polyolefin. The present invention relates to a method for anti-corrosion coating of steel pipes, which has a second layer made of a composition and a third layer made of a polyolefin, which have excellent adhesive strength and excellent water resistance retention properties.

(Problems with conventional technology) Conventionally, chemically inert,
Olefin resins such as polyethylene and polypropylene are used, which have excellent mechanical properties and processability, as well as excellent corrosion resistance and durability. More specifically, in order to increase the adhesion between the olefin resin and the outer surface of the steel pipe, an adhesive layer such as a modified polyolefin into which a polar adhesive functional group such as a carboxylic acid group or its anhydride group is introduced is used. A method of coating with olefin resin is used. This method fuses the modified polyolefin at a temperature higher than its softening temperature, which allows it to be integrated with the olefin resin and exhibits high adhesive strength, as well as a certain degree of adhesion to the outer surface of steel pipes. . Since this steel pipe with external anti-corrosion coating using olefin resin is coated by continuous extrusion, the adhesive interface between the outer surface of the steel pipe and the coating layer is not exposed to the external environment. It has excellent hot water resistant adhesive strength.

However, if defects occur in the coating layer due to impact during handling, etc., there is no complete repair method (the functional groups that exhibit adhesive properties of modified polyolefins are mainly carboxylic acid groups or their anhydride groups). As a result, the water-resistant adhesive strength, especially the hot water-resistant adhesive strength, is at an insufficient level, and as a result, the adhesion of the coating material decreases from the defective part, making it ineffective as an anti-corrosion coating material. This has become a big problem, for example, in buried steel pipes, etc., for which the demand for high-temperature durability has increased in recent years.

(Problems to be Solved by the Present Invention) The problem to be solved by the present invention is to improve the decrease in adhesive strength caused by defective portions that occurs in the above-mentioned conventional coating method.

(Means for solving the problem) This problem is mainly caused by polyolefins containing one or more functional groups of carboxylic acid groups or dicarboxylic acid anhydride groups as modified polyolefins, and polyolefins containing one or more functional groups in one molecule. This problem is solved by using an adhesive resin composition formed by melt-mixing an epoxy resin having an internal epoxy group and at least one external epoxy group.

That is, according to the research conducted by the present inventors, steel pipes that have been subjected to conventional surface treatments such as cleaning and blasting are heated at 150'C.
After preheating as above, first coat the epoxy powder coating as the first layer, then fusion coat the adhesive resin composition layer as the second layer while the epoxy powder coating loses its fluidity, and then immediately coat the adhesive resin composition layer as the second layer. By fusion-coating the polyolefin layer as the third layer, a high and stable adhesive force can be obtained due to the cross-linking polymerization reaction at the interface between the epoxy resin layer and the adhesive resin composition layer, and the adhesive resin composition layer It has been found that the adhesive strength can be improved by integrating the polyolefin layer and the polyolefin layer, thereby improving the decrease in adhesion from the end face of the coating layer at defective areas and the like.

(Structure and operation of the invention) In the present invention, the steel pipe on which the coating is formed is not particularly limited in size. The outer surface of this steel pipe is preferably degreased and blasted in advance before being coated with epoxy powder paint, and after blasting, acid treatment such as hydrochloric acid or phosphoric acid,
It may be treated with chemical conversion treatment using phosphate or chromate.

As the epoxy powder coating used in the present invention, any of those conventionally used in this type of field can be used, and there are no particular restrictions, but epoxy powder coatings for pipe coating are preferably used. Particularly preferred is an amine-curing epoxy powder coating for pipe coating.
It is suitable because it has excellent hot water resistant adhesive strength. For example, an epoxy resin has an epoxy equivalent of 750 g/eq.
An example of this is an epoxy powder coating that is a bisphenol A type epoxy resin, is an aromatic amine hardener system, and contains 35% filler system.

The modified polyolefin used in the present invention is, in principle, a polyolefin in which a carboxylic acid group or its anhydride group has been introduced.There are various types of olefin in this case, but ethylene is used as a representative example for explanation. I decided to. Modified polyethylenes include ethylene copolymers obtained by copolymerizing ethylene with unsaturated monomers having carboxylic acid groups or dicarboxylic anhydride groups, and unsaturated polyethylenes having carboxylic acid groups or dicarboxylic anhydride groups in polyethylene. Graft polyethylene obtained by graft copolymerizing a saturated monomer can be used. As the unsaturated monomer in this case, monomers having one or more carboxylic acid or its anhydride group and one or more unsaturated groups can be widely used, and more specifically monobasic or monobasic monomers can be used. Polybasic unsaturated carboxylic acids or their anhydrides are preferably used, and specific examples include acrylic acid, methacrylic acid, maleic acid, itaconic acid, fumaric acid, maleic anhydride, fumaric anhydride, itaconic anhydride, etc. I can do it.

In order to obtain the above-mentioned modified polyethylene, in the case of an ethylene copolymer, ethylene and the above-mentioned unsaturated monomer are copolymerized by various known methods such as high-pressure radical polymerization, solution polymerization, and emulsion polymerization. In the case of graft polyethylene, it can be obtained by various known methods such as radical polymerization, radiation grafting, and ion grafting.

In the case of ethylene copolymers, the unsaturated monomer content in these modified polyethylenes is 0.
．． 05 to 10 mol%, preferably 0.1 to 8 mol%, and in the case of grafted polyethylene, 1 g of polyethylene
The unsaturated monomer is used in an amount of 10-5 to 2 x 104 g equivalent, preferably 2 x 10-5 to 10-3 g equivalent. If the content of the unsaturated monomer is too small, the compatibility with the epoxy resin will be poor, and the resulting composition will be difficult to exhibit sufficient adhesive strength. On the other hand, if the content is too large, problems arise such as a decrease in the adhesion with the unmodified polyolefin and a decrease in water resistance due to a decrease in the compatibility of the obtained composition with the unmodified polyolefin.

Although the case where ethylene is used as the olefin has been explained above, in the present invention, olefins other than ethylene such as propylene, butylene, etc. can also be used, and olefins having usually 6 or less carbon atoms, preferably 4 or less carbon atoms can also be used. It can be used.

In addition, the optimal content of unsaturated monomers having carboxylic acid groups or their anhydride groups used for modification may differ slightly depending on the type of olefin used, but the content range is within the range of the polyethylene mentioned above. Same as in case.

Next, the epoxy resin used in the present invention must have one internal epoxy group and at least one terminal epoxy group in one molecule, and in particular, the internal epoxy group must be a cyclopentene oxide group, a cyclohexene oxide group, etc. It is preferable to use a tricyclohexene oxide group in which the terminal epoxy group is a glycidyl ether, glycidyl ester, glycidyl amine or ethylene oxide.

In the present invention, the internal epoxy group and the terminal epoxy group respectively mean the following. That is, an internal epoxy group is an epoxy group that exists inside the molecule rather than at the end,
In addition, due to the steric hindrance of the molecular structure, it has low reactivity with nucleophiles and high reactivity with electrophiles,
Further, the terminal epoxy group is an epoxy group present at the end of the molecule, and has a molecular structure that does not sterically hinder nucleophilic reagents.

In the epoxy resin of the present invention, one having one internal epoxy group is used, and in the case of a terminal epoxy group, one containing one or more is used.

The mixing ratio of the modified polyolefin and the epoxy resin in the adhesive resin composition of the present invention is preferably 0.2 to 1.5 equivalents of the internal epoxy group per equivalent of the carboxylic acid group or dicarboxylic acid anhydride group of the modified polyolefin. Preferably 0.6~
The mixing ratio is set to 1.2 equivalents. If the amount of the internal epoxy group is less than 0.2 equivalents, the adhesive strength and water resistance will decrease, but if it is more than 1.5 equivalents, the adhesive strength with unmodified polyolefin will decrease and the layer separation of unreacted epoxy resin will occur. It is easy to wake up and is not desirable.

Next, in order to obtain the adhesive resin composition of the present invention, the above-mentioned raw materials are melt-mixed by appropriate means such as a mixing roll, a kneader, an extruder, a mixing pot, etc., and the mixing temperature is preferably 100 to 160'C, especially Preferably 100-
The temperature is 140°C. If the mixing temperature is too high, the ring opening of the terminal epoxy group may increase the melt viscosity or cause gelation, and if it does not reach 100"C, the viscosity is too high and melt mixing is difficult.Mixing time is appropriately determined depending on the reactivity of the raw materials used and the mixing temperature, but generally ranges from several minutes to several tens of minutes.The adhesive resin composition of the present invention may contain a free filler depending on its use. , pigments, flame retardants, foaming agents, antifoaming agents, antioxidants, leveling agents, plasticizers, etc. can be added within ranges that do not impair the effects of the present invention.These additives are usually modified with epoxy resins. It is usually added before mixing and melting the polyolefin.

The third layer of polyolefin used in the present invention is fused to the coated surface of the adhesive resin composition, and is an olefin homopolymer or a mutual copolymer of these, and is usually a lining olefin. It is used as a resin.

The olefin in this case usually has 6 or less carbon atoms,
For example, ethylene, propylene, butylene, tributylene, pentene, etc. can be exemplified, and preferred examples include ethylene and propylene. The olefin also includes diene monomers such as butadiene and triprene. Specific examples of preferably used lining polyolefins include, for example, low, medium or high density polyethylene, polypropylene, ethylene-
Examples include propylene copolymers, and two or more of these can be used in combination.

In addition, the olefin resin for lining is generally 10g/
It is preferred to have a melt index of 10 minutes or less, preferably Ig/10 minutes or less. This polyolefin may further contain coloring pigments, extender pigments, reinforcing materials, and other additives such as dispersion aids such as silane coupling agents, ultraviolet absorbers, antioxidants, etc., as required.

The method of anti-corrosion coating of steel pipes according to the present invention involves first subjecting the outer surface to a normal surface treatment, preferably cleaning and blasting, and then pickling or chemical conversion treatment as required.
Preheat to 0″C or higher, preferably 150 to 250″C,
Apply epoxy powder paint. The pickling and chemical conversion treatment itself at this time may be carried out by various conventionally known means, and the pickling may be carried out using hydrochloric acid or phosphoric acid, and the chemical conversion treatment may be carried out using various phosphates or chromates. Processing can be cited as a representative example.

The method for applying this epoxy powder coating is not particularly limited, and various conventional epoxy powder coating methods can be used. In addition, the preheating temperature of the steel pipe mentioned above varies depending on the epoxy powder coating and coating method used, but it is generally 150°C.
Below 'C, the melt adhesion of the epoxy powder coating becomes insufficient and the adhesion is reduced. Moreover, if the temperature exceeds 250°C, the epoxy coating film and polyolefin layer will deteriorate.

There is no particular limit to the coating film thickness of this first layer of epoxy powder coating, but the thickness is usually 50 to 200 μm or less.
It is about μm.

The steel pipe coated with the epoxy powder coating is then fused and coated with an adhesive resin composition layer as a second layer while the epoxy powder coating does not lose its fluidity. At this time, if the adhesive resin composition layer is fused and coated after the fluidity of the epoxy powder coating has disappeared, the crosslinking polymerization reaction at the interface between the epoxy resin layer and the adhesive resin composition layer is insufficient. Therefore, the high hot water resistant adhesive strength that is a feature of the present invention cannot be obtained.

Therefore, the adhesive resin composition can be applied to the steel pipe coated with the epoxy powder coating by an appropriate means such as powder, pellet, tape, film, or sheet while the epoxy powder coating does not lose its fluidity. It can be fused in a shape such as a shape or a tube. The fusion method varies depending on the shape, size, heat capacity, etc. of the steel pipe, but examples include extrusion coating using a cross rad tubing die or T die, fluidized dipping method, powder scattering method, powder electrostatic coating method, etc. These include powder fusion methods such as powder deposition method and powder injection method. Then, to fuse the composition, a steel pipe coated with epoxy powder coating was
If kept at a temperature of 50°C or higher, the crosslinking polymerization reaction between the epoxy group or (and) curing agent in the epoxy powder coating and the epoxy group or (and) adhesive functional group in the adhesive resin composition will proceed easily. This is preferable because the mutual adhesion between the two coating films is improved. The film thickness of the composition is approximately 50 to 200 mm.
μm is suitable. Finally, the anticorrosive coating method of the present invention is completed by fusing a polyolefin layer as a third layer onto the fusion coating of the adhesive resin composition.

At this time, the polyolefin layer can be formed in the same manner as the adhesive resin composition, and the film thickness is 0.5 to 5M.
range is suitable. In the case of extrusion coating, two layers may be extruded and integrated together with the above-mentioned adhesive resin composition in advance using a cross-layer die or the like, and this may be fused to a steel pipe coated with an epoxy powder coating.

(Example) The effects of the present invention will be explained below using examples.

The following parts refer to parts by weight. In addition, M■ hereinafter refers to the melt flow index in JIS K6760, which is the flow rate for 10 minutes at 190° C. under a load of 2160 g, and the unit is g/10 minutes. Further, as a reference example, an example of manufacturing an adhesive resin composition will be given. However, in the following reference example, the adhesive resin composition used in the present invention was obtained by melting the modified polyolefin and epoxy resin shown below at a resin temperature of 120° C. for 10 minutes using a mixing roll.

Reference Example 1 (Adhesive resin composition A) Modified polyethylene: 95 mol% ethylene and 5 mol% acrylic acid
Copolymerized polyethylene (M,
I = 1.5 g / 10 min -1 density 0.94)
: 100 parts Epoxy resin: Vinyl cyclohexene dioxide having cyclohexene oxide as an internal epoxy group and ethylene oxide as a terminal epoxy group Reference Example 2; (Adhesive resin composition B) Modified polyethylene resin in adhesive resin composition A: 101 Epoxy resin: 18 parts Reference Example 3 (Adhesive resin composition C) Modified polyethylene resin: Copolymerized polyethylene obtained by copolymerizing 95 mol% ethylene and 5 mol% maleic acid (M, I = 1. 0 g/10 min, density 0.93):
100 parts Epoxy resin: Vinyl cyclohexane dioxide: 7 parts Reference example 4 (Adhesive resin composition D) Modified polyethylene resin: Graft polyethylene obtained by graft copolymerizing 10-3 g equivalent of acrylic acid per 1 g of polyethylene (M, I = 1.0 g710 min, density 0.92):
100 parts Epoxy resin Nidiclopentene oxide monoglycidyl ether = 7 parts Reference example 5 (Adhesive resin composition E) Modified polyethylene resin: Graft polyethylene (M, I = 1.5 g/10 min, density 0.92
): 100 parts Epoxy resin dichloropentene oxide monoglycidyl ether: 7 parts Reference example 6 (Adhesive resin composition F) Modified polyethylene resin: Grafted polyethylene CM obtained by graft copolymerizing 5 x 10-3 g equivalent of maleic anhydride per Ig of polyethylene , I=1.5 g/10 min, density 0.92): 100 parts Epoxy resin: 27 parts vinylcyclohexene dioxide Example 1 The outer surface of a steel pipe with an outer diameter of 115 mm, an inner diameter of 109 mm, and a length of 400 mm was degreased and plasted. After that, it was heated to 18
Preheat to 0℃, apply epoxy powder paint in an electrostatic coating booth to a film thickness of 100 μm, and before the fluidity of the epoxy powder paint disappears, glue the steel pipe while rotating it with a horizontal gourd. Coating the resin composition A with a thickness of 100 μm,
Immediately add lining polyethylene (M, I =
0.5 g/l 0 min, density 0.92) in the same manner as 2
It was coated with a thickness of about 100 ml.

Further, after applying the epoxy powder coating, the steel pipe was kept at a temperature of 160° C. or higher in an insulating furnace until the polyethylene coating was completed. This operation can be expressed in a flow sheet as shown in Figure 1. However, in Figure 1, (1) is an induction heating furnace, (2) is an electrostatic coating booth, (3) is a gouer for coating adhesive resin compositions, (4) is a gooey for coating polyolefin, and (5) is a gooey for coating polyolefin. It is a steel pipe. Further, (6) indicates a heat retention furnace.

Example 2 A steel pipe was coated in the same manner as in Example 1 using adhesive resin composition B.

Example 3 A steel pipe was coated in the same manner as in Example 1 using adhesive resin composition C.

Example 4 A steel pipe was coated in the same manner as in Example 1 using the adhesive resin composition.

Example 5 Steel pipe coating was carried out in the same manner as in Example 1, using adhesive resin composition E and preheating to 240° C. using an induction heating furnace.

Example 6 A steel pipe was coated in the same manner as in Example 5, using adhesive resin composition F and setting the temperature of the steel pipe to 200°C.

Comparative Example 1 In Example 1, after the epoxy powder coating was applied and the coating film was completely cured, the same steel pipe coating was performed.

Comparative Example 2 A steel pipe was coated in the same manner as in Example 1 without applying the epoxy powder coating.

Comparative Example 3 In Example 1, ethylene 9 was used as the adhesive resin composition.
Copolymerized polyethylene (M, I = 1.5 g/10 min, density 0.94) obtained by copolymerizing 5 mol% of acrylic acid and 5 mol% of acrylic acid was used alone, and similar steel pipe coating was performed. Ta.

Comparative Example 4 In Example 1, graft polyethylene (M) was prepared by graft-polymerizing 10-3 g equivalent of acrylic acid per 1 g of polyethylene as the adhesive resin composition.

1 = 1.0 g/10 min, density 0.92) was used alone for similar steel pipe coating.

Comparative Example 5 A steel pipe was coated in the same manner as in Example 1 without using the adhesive resin composition.

A test piece of an appropriate size was cut out from each of the steel pipes of the above Examples and Comparative Examples, slits reaching the steel pipe surface were made at 10 mm intervals in the length direction of the steel pipe, and the test pieces were subjected to the following tests. The results are shown in Table 1.

[Initial adhesive strength]

The coating layer was formed and the tensile speed was 50 using a tensile tester.
Peel adhesion strength at 180° beer was measured in mm/min.

[Adhesion strength after immersion in fresh water]

After the test pieces were immersed in fresh water for 2 and 20 days, the peel adhesion strength was measured in the same manner as above.

〔effect〕

As is clear from the Examples above, the method for anticorrosion coating of steel pipes according to the present invention shows that the coating has excellent adhesion, water resistance, and hot water resistance.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet showing an example of implementing the present invention. 1... Induction heating furnace 2... Electrostatic coating booth 3... Gui 4... Gui 5... Steel pipe (or more)

Claims (5)

[Claims] (1) First layer of epoxy powder coating on the outer surface of the steel pipe,
A method for anti-corrosion coating of steel pipes, in which a second layer made of an adhesive resin composition mainly composed of modified polyolefin and a third layer made of polyolefin are sequentially formed. An epoxy powder coating to form a layer is applied, an adhesive resin composition mainly composed of a modified polyolefin to form a second layer is fused and coated while the epoxy powder coating loses its fluidity, and then immediately The adhesive resin composition is characterized in that the polyolefin layer as the third layer is fused and coated, and the adhesive resin composition mainly contains a modified polyolefin.
It is characterized by being formed by melt-mixing a modified polyolefin containing one or more kinds of functional groups and an epoxy resin having one internal epoxy group and at least one terminal epoxy group in one molecule. A method for anticorrosive coating of steel pipes. (2) The epoxy powder coating is an amine-curing type, and the modified polyolefin of the adhesive resin composition is a copolymer of ethylene and an unsaturated monomer having a carboxylic acid group or a dicarboxylic acid anhydride group. The anticorrosive coating method according to claim 1, wherein the content of the unsaturated monomer is 0.05 to 10 mol%. (3) The epoxy powder coating is an amine-curing type, and the modified polyolefin of the adhesive resin composition contains an unsaturated monomer having a carboxylic acid group or a dicarboxylic acid anhydride group in polyethylene at 10^-^ per gram of polyethylene. 5~2×1
The anticorrosive coating method according to claim 1, which is modified polyethylene obtained by grafting at a ratio of 0^-^3g equivalent. (4) Claim 2 or 3, wherein the internal epoxy group of the epoxy resin is a cyclopentene oxide group, a cyclohexene oxide group, or a tricyclohexene oxide group, and the terminal epoxy group is a glycidyl ether, a glycidyl ester, or a glycidyl amine. Anti-corrosion coating method. (5) The preheating temperature of the steel pipe is 150°C to 250°C, and the temperature is 150°C from the time of preheating to the time of fusion coating with polyolefin.
The anticorrosion coating method according to any one of claims 1 to 4, characterized in that the heat retention is carried out at a temperature higher than that.